Barrier coatings (layers) which prevent, reduce, or inhibit the permeation of a selected substrate with a gas, vapor, chemical and/or aroma have been widely described, and such coatings are used in a variety of industries, e.g., the packaging industries, automobile industries, paint industries, tire industries etc. Typical barrier materials used in coatings include polyesters, PVDC, polyurethanes, acrylic polymers, etc.
It is well known that the barrier properties of a polymer can be improved by the addition of impermeable plate like structures. When the plates are oriented perpendicular to the diffusion (permeation) direction, the diffusing molecules must go around the plates. This leads to significant reductions in the permeability of the polymer. See, for example, E. L. Cussler et al, J. Membrane Sci. 38:161-174 (1988); W. J. Ward et al, J. Membrane Sci., 55:173-180 (1991); Chang, J. et al, Journal of Applied Polymer Science, Vol. 84, 2294 (2002); Yano, K. et al, Journal of Polymer Science A: Polymer Chemistry, 35, 2289 (1997); Lan, T. et al, Chem. Mater. 6, 573 (1994); Messersmith, P. B. and Giannelis, E. P, Journal of polymer Science A: Polymer Chemistry 33,1047 (1995); U.S. Pat. Nos. 4,528,235; 4,536,425; 4,911,218; 4,960,639; 4,983,432; 5,091,467; and 5,049,609; and International Patent Application No. WO93/04118, published Mar. 4, 1993, among others.
Control of permeation using relatively low aspect ratio platelets, at low concentrations, and thermoplastically processed at high shear rates has been previously disclosed. See, for example, E. L. Cussler et al, J. Membrane Sci. 38:161-174 (1988); L. E. Nielsen, Journal of Macromolecular Science, Chemistry A1,929, (1967); R. K. Bharadwaj, “Modeling the Barrier Properties of Polymer-Layered Silicate Nanocomposites”, Macromolecules 34, 9189 (2001); G. H. Fredrickson and J. Bicerano, “Barrier properties of oriented disk composites”, Journal of Chemical Physics 110, 2181 (1999). These conditions lead to relatively small improvements in the barrier properties of the polymer. This is because the reduction in permeability varies rapidly with the aspect ratio and the concentration of plates when the plates are well aligned. If the plates are not well aligned, the reductions in permeability are further reduced. The targeted application of these earlier efforts was not coatings, but a bulk polymer with improved barrier and/or mechanical properties.
Use of platelet fillers in coating formulations is also well known. Most often, they have been used in paints to modify the rheology, enabling the production of no-drip paints. These platelet fillers are typically exfoliated silicates with aspect ratios of 50 or less. They form a house of cards type structure in the coating suspension that gives a gel like property to the paint (or coating) when it is not undergoing any shear. Generally, these structures do not have the optimally aligned plates to significantly reduce the permeability of the coating.
Exfoliated silicates have been used to produce nanocomposite coatings by several methods. The most widely used has been by combining a dissolved polymer with exfoliated filler. Water-soluble polymers such as polyvinyl alcohol (PVOH) have been combined with water exfoliated filler such as vermiculite. See, for example, Japan patent 11-246729, Sep. 14, 1999, “Gas-Barrier Poly(vinyl alcohol)/poly (acrylic acid) Compositions and their Laminates and Shaped Articles”. Sumitomo Chemical Co., Ltd. Polycarbonate dissolved in toluene has been combined with organically functionalized filler and reportedly forms good barrier coatings. See, for example, W. J. Ward et al, “Gas Barrier Improvement Using Vermiculite and Mica in Polymer Films”, Journal of Membrane Science, 55:173-180 (1991). Other polymers have also been made into barrier coatings by dissolving them in a solvent, and using an organically functionalized filler in an effort to improve the barrier properties. See, for example, Yano, K., et al, “Synthesis and properties of polyimide-filler hybrid composites”, Journal of Polymer Science A: Polymer Chemistry, 35, 2289 (1997).
Processes for utilizing emulsion polymerization procedures for preparing aqueous polymer/clay nanocomposite dispersions are disclosed in U.S. Pat. No. 6,838,507; U.S. patent applications 2005/0059769 and 2002/0086908 (all to Rohm and Haas). The disclosed processes include both in-situ polymerizations in the presence of at least partially exfoliated unmodified clays as well as admixtures of polymer dispersions with at least partially exfoliated unmodified clay dispersions. The disclosed nanocomposite dispersions are useful for preparing a variety of materials, such as coatings, adhesives, caulks, sealants, plastics additives, and thermoplastic resins. Processes for preparing polymer clay nanocomposite powders and use of these powders as plastic resin and plastics additives are also disclosed.
The process in the '507 patent provides a first aqueous reaction mixture comprising at least one ethylenically unsaturated monomer and a second aqueous reaction mixture comprising an at least partially exfoliated aqueous clay dispersion having at least one unmodified clay and at least one ethylenically unsaturated monomer. The two reaction mixtures are combined and polymerized. One of the monomers is an acid containing monomer.
Additional processes for utilizing emulsion polymerization procedures for preparing aqueous nanocomposite dispersions are disclosed in U.S. Pat. No. 6,759,463 and U.S. patent application 2002/0058740 (both to Rohm and Haas). In these cases, the disclosed processes include both in-situ polymerizations in the presence of at least partially exfoliated, lightly modified clays as well as admixtures of polymer dispersions with at least partially exfoliated, lightly modified clay dispersions. The clays in this invention are hydrophobically modified through the use of an agent such as a surfactant, silane, or other modifier. The agents may include amino acid surfactants, alkylammonium ion surfactants, silanes, aminomethylstyrene, living free radical polymerization initiator (“LFRP”), a polymerizable surfactant or by an acid ion exchange process. The acid ion exchange process comprises adding an ion exchange resin to the clay.
Another method used to form nanocomposites by incorporating the exfoliated filler into the monomer before polymerization is found in U.S. Pat. No. 4,472,538 “Composite Material Composed of Filler Mineral and Organic High Polymer and Method for Producing the Same”, Sep. 18, 1984; U.S. Pat. No. 4,889,885 “Composite Material Containing a Layered Silicate”, Dec. 26, 1989. Other techniques incorporating exfoliated clay into monomer droplets before the emulsion polymerization for elastomeric polymers are described in PCT Patent No. WO 97/00910, Jan. 9, 1997, “Polymer nanocomposite Formation by emulsion Synthesis”, Exxon Research and Engineering Co. Methacrylate monomer was combined with exfoliated filler in aqueous dispersion prior to its polymerization into a nanocomposite. See, for example, Lee, D. C. and Jang, L. W., Journal of Applied Polymer Science, Vol. 61, 1117-1122 (1996). These methods were designed to help make bulk nanocomposites for thermal processing; none of the methods led to practical coating formulations.
There are several examples of using an aqueous dispersion of exfoliated filler with an aqueous dispersion of polymer to form a nanocomposite. Most of that work used elastomeric polymers in suspension. See, for example, Wu, Y-P et al, “Structure of Carboxylated Acrylonitrile-Butadiene Rubber (CNBR)-Filler Nanocomposites by Co-coagulating Rubber Latex and Filler Aqueous Suspension”, Journal of Applied Polymer Science, 82, 2842-2848 (2001); Wu, Y-P et al, “Structure and Properties of Nitrile Rubber (NBR)-Filler Nanocomposites by Co-coagulating NBR Latex and Filler Aqueous Suspension”, Journal of Applied Polymer Science, 89, 3855-3858 (2003); Varghese and Karger-Kocsis, “Natural rubber-based nanocomposites by latex compounding with layered silicates”, Polymer (in press) (2003); Feeney et al, U.S. Pat. No. 6,087,016, “Barrier Coating of an Elastomer and a Dispersed Layered Filler in a Liquid Carrier”, Jul. 11, 2000; Feeney et al, U.S. Pat. No. 6,232,389, “Barrier Coating of an Elastomer and a Dispersed Layered Filler in a Liquid Carrier and Coated Articles”, May 15, 2001; Goldberg et al, “Nanocomposite Barrier Coatings for Elastomeric Applications”, Materials Research Society, Symposium T: Polymer nanocomposites, paper T4.7, (April 2002); and Goldberg et al, “Elastomeric Barrier Coatings for Sporting Goods”, ACS Rubber Section, Apr. 29, 2002, paper 17, published in Rubber World, vol. 226, No. 5, p 15 (August 2002). These references do not employ ion exchange techniques to make the filler surface more compatible with the polymer because it typically causes the filler to fall out of aqueous suspension.
In order to form a nanocomposite from a combination of polymer spheres and filler platelets, one needs significant flow and deformation of the polymer. Thus it was not expected that this approach would work with more rigid, non-elastomeric polymers. The only example found that tried this approach in a non-elastomeric polymer is described in Oriakhi and Lerner [“Poly(Pyrrole) and Poly(Thiophene)/Filler Nanocomposites via Latex-Colloid Interaction”, Materials Research Bulletin, 30, 723-729(1995)]. That method involved forming separate aqueous polymer latex and aqueous exfoliated clay suspensions. The latex was washed repeatedly before combining with the exfoliated clay in order to remove stabilizers and surfactants. Mixing the suspensions did not lead to stable coating suspensions, but rather coagulating mixtures where the nanocomposite came out of suspension. These mixtures of clay and polymer dispersed in water could not be used as a coating formulation and are therefore very different from the invention described in this patent.
The approach used by us and described in this patent differs from the above art in that it leads to stable coating formulations that can be applied to a range of articles in order to form an acrylic nanocomposite coating. The nanocomposite forms during the drying process which is well below the melt temperature of the polymer. Thus it is clear that the polymer particles undergo significant deformation during drying.